Will we be able to use your awesome tool to edit Dwarven likes/dislikes/preferences? And appearance values? Relationships? Pets?
Edit: Also, will I be able to modify the number of dorfs in my fortress at any given point? Say I want to start with 20 or so...will I be able to just add them into the save?
You will be *hopefully* (assuming it works when I get round to writing it) be able to edit traits. Eventually, though they're not currently displayed, appearance could be added for display and editing (again, in theory). Setting pets and Relationships is one of those "I suppose it may be technically possible, but don't expect it any time soon" things. TBH without doing some research and talking to petrix, I can't really say whether it's doable or not.
Editing creatures is working though, so you can edit creatures that are already pets. You can also do things like make marauding cats or tame demons. In conjunction with Darius' mod linked above, it should allow them to work for you.
The number of starting dwarves can be changed during embark by a number of tools, though currently not this one. As for adding dwarves while playing, I'm not sure that's feasible tbh.
Which platforms will this work for?
Windows and Linux. From what I understand, DFHack for Mac isn't really in a usable state right now. So if any Mac users want to use it, best you help get DFHack working on Mac!
I never used Dwarf Companion, so what does skill level editing mean? does it mean I could turn someone into a High Master engraver instantly?
Yes, exactly. You could also make them level 200 if you wanted to, though in game they would still be displayed as legendary, the levels do have an effect. The highest I've level legendary I've seen so far without hacking is 29.